In conventional video systems such as television, a video image is produced by scanning an electron beam in a raster format across the face of a tube having phosphors which glow with either a white light or color, depending upon the system. Because the phosphors only glow for a predetermined amount of time, the images are relatively volatile and must be frequently rewritten.
In the most common example, an NTSC television format includes a raster scan of 525 horizontal lines per video frame. To give the illusion of a single image, many of these frames are combined sequentially in time thereby allowing the eye to integrate the combination and perceive moving images on a screen. Usually, the frame rate is 30 frames per second, with two interlaced fields forming a frame. The interlace technique, where one half of the 525 horizontal lines is shown in one field followed by the other half of the horizontal lines in the next field, is to reduce the phenomenon of flicker. Flicker is caused by the on/off nature of the video signals and produces an annoying artifact at the field frequency.
It is known that increasing the frame rate will reduce the perception of flicker, but such requires additional bandwidth to transmit the signal and additional equipment to decode and display the video signals. It is known that much of the bandwidth of a television signal, as much as 60%, is required for this flicker fusion due to the high repetition frame rate.
The phenomenon of flicker is also noted in optical projection systems which are used in commercial movies. For example, a standard movie projector displays 24 frames of a film per second, and these frames in turn are each transilluminated three times. This system, therefore, gives 72 flickered images per second to maintain flicker fusion.
An image which did not flicker would eliminate these high speed repetitive showings of the image because there would be no need for flicker fusion. Much of the time, the image shown in a video environment does not change from frame to frame. It is only when an element of the image is moving does the frame itself change. However, the frame rate of motion fusion is much less than flicker fusion. The motion fusion rate can be as low as 11 frames per second, and thus a system which imaged without flicker could be designed to operate at this lower repetition rate. This would reduce the bandwidth for transmitting video signals with present signal formats by as much as 60%. However, a flickerless video system envisions a nonvolatile screen display other than the present phosphor display screens.
The video technology is advancing to where large screen optical projection systems are rivaling commercial movie theaters. These large screen optical projection systems are more common today and generally include a video monitor with a projection system to receive an image from a cathode ray tube screen and to magnify it by optical means onto a larger screen. However, these system are today somewhat limited in final screen size because of cost and efficiency. To build a cathode ray tube for the initial image which is bright enough, or with enough image definition, to permit significant magnification without substantial reduction in image quality is extremely difficult and expensive. Further, the conventional cathode ray tube is inherently inefficient for such projection because of the way the image is formed. The phosphors of a cathode ray tube screen emit scattered, incoherent light. It is difficult to capture much of the energy from this type of image for projection, which creates a consequent reduction in the brightness of the final image. There is presently no relatively inexpensive optical projection system for projecting a video image from a cathode ray tube screen to movie screen size.
However, the distribution of video images and their initial formation are much easier with cathode ray tube technology than with the image medium of film. The distribution of video images by means of over-the-air transmission, video disk or cassette is well developed and the technology is highly efficient. The imaging of a cathode ray tube screen where monochrome or color images can be made by a raster scan of phosphors on the display screen of the tube is also relatively efficient and much more effective than transmitting illumination through a celluloid based film. The production of video programs on film requires a complex distribution system and creation process. It is much easier to make, combine and edit a video tape or disk than it is to perform the same operations for a film of comparable duration. Moreover, video special effects can be more easily incorporated into video tape or disk than on film.
Therefore, it would be highly desireable to provide an optical projection system which incorporates the efficient video technology of the cathode ray tube while replacing the inefficient projection and magnification technology of the phosphor screen display. It would also be highly desirable if such optical projection system included a display screen which was nonvolatile to allow such system to operate at the motion fusion rate without flicker.
An image medium which possesses many desirable optical characteristics for these purposes are electro-optic materials. Electro-optic materials provide a medium by which electrical information can be transformed into optical information or by which optical functions can be performed upon command from an electrical signal. These electro-optic materials are typically single crystals, such as KDP, LiNbO.sub.3, LiTaO.sub.3, and SBN or even liquid crystals.
Electro-optic ceramics, particularly including PLZT, Pb.sub.x La.sub.y (Zr.sub.z Ti)O.sub.3, are one of the newest classes of electro-optic materials finding increasing use in such applications as optical shutters, modulators, displays, color filters, image storage devices, and linear gate arrays for optical processing. Their success in these applications thus far has been largely due to a number of factors, including (1) high optical transparency, (2) high electro-optic coefficients, (3) fast response time, (4) high electrical resistivity, (5) direct current (DC) operation, (6) low power consumption, (7) memory capability, (8) good property uniformity over large areas, (9) moisture insensitivity, and (10) low cost. Foremost among these characteristics is high optical transparency since many of the other properties cannot be effectively utilized in electro-optic devices unless the material is capable of transmitting a large portion of the incident light. (See Haertling, G., Chap. 7, Electro-optic Ceramics and Devices in Electronic Ceramics, Ed. Levinson, Marcel Decker, Inc., 1988).